Ion selective electrodes (ISE's) have widespread applications in the fields of biology, chemistry, and medicine. These electrodes provide a useful analytical technique for detecting and measuring the concentration of a particular ion species in solution. The applications of ISE's are numerous, including biomedical research, clinical testing, industrial pollution testing, and chemical-process control.
In clinical medicine, ISE's are important in the diagnosis and treatment of diseases due to their ability to measure ion concentrations in blood, serum, plasma, cerebral spinal fluid, and urine samples. Ions commonly measured in clinical testing include cations and anions. For example, chloride ion levels in bodily fluids are characteristic of certain electrolyte and metabolic disorders including cystic fibrosis, the most common serious genetic disorder in the United States. Similarly, measurements of calcium ion concentration levels are used in the diagnosis of endocrine and renal diseases and in monitoring diseases like cancer. Therefore, it is important that ion concentrations be accurately measured.
Currently, electrolyte analyzers have been developed based on ion-selective electrode technology. In such analyzers, an ISE and an external reference electrode pair are immersed simultaneously in a sample solution. An electrical potential is developed between the electrodes, due to the presence of the ion to which the ISE is sensitive. By measuring this potential, the concentration of the ion can be determined.
Early designs of ISE's comprised an ion selective membrane affixed to the lower opening of a plastic electrode body. The electrode body has an inner electrolyte solution and a reversible internal reference electrode sealed within. This design has several disadvantages including low durability and low reproducibility.
In more recent designs, solid state ion selective electrodes have been developed which utilize a solid ion selective membrane. However, a problem encountered with electrodes of this type is weakness in the physical adhesion between the ion selective membrane and the electrode body. This can alter the membrane potential resulting in inaccurate ion measurements. In addition, electrodes having an internal reference electrode and solution are relatively delicate instruments, thus requiring frequent maintenance.
In order to overcome these problems, solid state ion selective membrane electrodes have been developed which eliminate the inner electrolyte solution and reference electrode. In this type of ISE, the liquid internal reference electrode has been replaced by a solid support which is electrically conductive. These electrodes include a direct electrical contact to the inner surface (the surface not in contact with the sample solution) of the ion selective membrane. The membranes of these electrodes commonly include a polycrystalline pressed pellet of solid electrolytes (made by compressing the solid electrolyte mixture at very high pressures). Additionally, these electrodes may have a silver or gold plating on the membrane inner surface for electrical contact to a voltmeter.
However, a disadvantage associated with these solid state electrodes is the difficulty in providing good physical contact between the ion selective membrane and the element to insure a direct electrochemical interaction. Without a sufficient interaction, variations can occur in the potential difference between the sample and the standard reference electrode resulting in inaccurate and inconsistent ion concentration measurements. Additionally, these types of electrodes tend to drift. Another disadvantage is the deterioration of the ion selective material resulting in the life of the electrode being shortened.
Conventional silver/silver chloride electrodes are commonly used to measure chloride ion concentrations. However, these electrodes have a variety of problems. One problem is that silver/silver chloride electrodes require frequent maintenance such as polishing and bleach cleaning, especially when these electrodes are exposed to many urine samples. Without such maintenance, these electrodes respond sluggishly and are a major cause of reference drifts. Additionally, inconsistencies in electrode performance can be attributed to variability in particle size of the metallic silver powder used to manufacture these electrodes.
For the foregoing reasons, there is a need for an all solid state ion selective electrode which exhibits high selectivity, excellent reliability and the ability to accurately detect and measure the concentration of ions in solution. Further, it would be advantageous for this ion selective electrode to have low impedance, be fast and stable in response to potential, and be able to maintain its performance for prolonged periods of time.